DND_Reborn
The High Aldwin
First, I said it was a "nearly pointless feature" [of the feat], not that the feat itself was pointless without this. That is an important distinction. (bold added for emphasis)
As I said, unless you have allies who can take advantage of the knock down you caused, the opponent can stand up on their turn before you benefit from the knock down. Now, if you want to keep allowing OA at disadvantage against you, I suppose that would be a way to deny them any real attack. There is nothing wrong with that and I hadn't thought of it, so it is more useful when used defensively. However, it offers you no benefit offensively otherwise and if you look at the rest of my post, it demonstrates a perfectly acceptable way to use it where it at least can be used offensively by you without the need for having an ally there to hit the target.
So, thanks for the idea on how to use it defensively at least, that does give it some more merit even as currently ruled.
Except Dave doesn't move to engage. He gets up and readies his attack for when Stan moves into his reach. Now, when Stan moves to engage Dave again, Dave gets his attacks before Stan. Also, you can at best assume a 50-50 chance for the knock down. As well, you're ignoring any feat Dave might have that would assist him, perhaps Sentinel, so if he does his with the OA, Stan isn't moving any more. At any rate, as I expressed above, this is a useful "maneuvering" way to employ Shield Master, which I thanked FrogReaver for.
Cool, then no worries, but you could be less of an ... about it.
From a power perspective there are a lot of pointless feats. I don't feel feats were well balanced in terms of power. They feel more about flavor than power to me. As such you get some utterly weak ones and some irreplaceably powerful ones.
That said knocking someone prone is still potentially beneficial to your allies and slows down the enemy from getting to your squishies. So even in the worst case interpretation, shield master isn't a pointless feat.
By the way consider this strategy in a 1v1.
Fighter 1: Attack, attack, shield bash Knock prone. move 20 ft away, taking an OA at disadvantage.
Fighter 2: Stand up, dash 20 ft ends turn adjacent
Fighter 1: repeat.
There's plenty of benefit to shield master in 1v1 combat even when you can only shove after. You just have to think tactically.
As I said, unless you have allies who can take advantage of the knock down you caused, the opponent can stand up on their turn before you benefit from the knock down. Now, if you want to keep allowing OA at disadvantage against you, I suppose that would be a way to deny them any real attack. There is nothing wrong with that and I hadn't thought of it, so it is more useful when used defensively. However, it offers you no benefit offensively otherwise and if you look at the rest of my post, it demonstrates a perfectly acceptable way to use it where it at least can be used offensively by you without the need for having an ally there to hit the target.
So, thanks for the idea on how to use it defensively at least, that does give it some more merit even as currently ruled.

5e system is built and designed around team play, do the assumption is there will be allies and elements are based on it.
Have you looked at how effective bardic inspire dice are sat levrls 1-2 if you are solo? How effective Inspiring leader is solo? Or the help action entirely solo?
Shield Master gives you three benefits, two of which are solo fine - they just affect you. It cannot have one with diminished capability while solo?
Now for your example, let's take two fighters at 5th level one-on-one.
Stan has Shield Master, Dave does not. Both are primarily sword xnd board types.
Stan takes two swings, 5th level then bonus shoves.
Say Dave goes down.
Stan steps away allowing Dave a disadvantaged AO. Moves 30 ft.
Dave can get up, but cannot close to melee this turn. Maybe he can juggle the interactions to throw something but that's hardly a match.
So one disadvantaged and one interaction juggled throw vs two normal melee swings.
"nearly a pointless feature"?
Now swap Dave for any number of beasts/monsters with less than 50' movement and reach combined - now they likley dont have a "throw option." Now every round you can bonus shove and succeed at the end is an exchange where you gain the edge of taking your two melee attacks at full and all the opponent gets is one disadvantaged AO.
"nearly a pointless feature"?
As one-third of a feat?
Even after limiting it to solo fights and ignoring its team play potential?
Gotcha. Sure thing.
Except Dave doesn't move to engage. He gets up and readies his attack for when Stan moves into his reach. Now, when Stan moves to engage Dave again, Dave gets his attacks before Stan. Also, you can at best assume a 50-50 chance for the knock down. As well, you're ignoring any feat Dave might have that would assist him, perhaps Sentinel, so if he does his with the OA, Stan isn't moving any more. At any rate, as I expressed above, this is a useful "maneuvering" way to employ Shield Master, which I thanked FrogReaver for.
As for your TWF run on...
" It is illogical to assume every time the character attacks they MUST strike with the main-hand first."
Yes, exactly, which is why "main hand" and off-hand are not in the 5e TWF rules. There is no limitation that requires either hand to start the sequence, just that the bonus action must be with a different light weapon in a different hand. That's all. Depending on styles and specifics, that bonus attsck may have no ability modifier but that's tied to its bonus action snd could apply to one hand on one turn and the other on the next.
Far as I can tell, your shortsword dagger whatever examples are all legal within std 5e rules and the only change your tule fir was allow a little different answer to "which attack loses the ability score bonus" **if** they lack the style that negates that.
Not really mind blowing?
Cool, then no worries, but you could be less of an ... about it.
